An integrated circuit is a complete electronic circuit, containing transistors, diodes, resistors, and capacitors, along with their interconnecting electrical conductors, contained entirely within a single chip of silicon. Integrated circuits continue to decrease in size, and the circuits they contain continue to increase in complexity. This increases the chances of defective chips resulting from a failed element or a defective conductor.
One way to reduce semiconductor scrap is to provide redundant elements on the integrated circuits. If a primary element is defective a redundant element can be substituted for that defective element. One example of an integrated circuit device which uses redundant elements is electronic memory. Typical memory circuits comprise millions of equivalent memory cells arranged in addressable rows and columns. By providing redundant elements, defective memory cells or columns can be replaced. Because the individual primary memory cells of a memory are separately addressable, replacing a defective cell typically comprises opening fuse-type circuits to `program` a redundant cell to respond to the address of the defective primary cell. This process is very effective for permanently replacing defective primary memory cells. For example, FIG. 1 illustrates a typical memory circuit where primary memory columns (PRIME.sub.0 to PRIME.sub.i) are selectively connected to data communication lines (DATA.sub.0 and DATA.sub.i). When a primary column is addressed via external address lines, the appropriate select signal (SEL.sub.0 to SEL.sub.i) is activated. If a primary column is determined to be defective, its select signal is forced to a permanent inactive state, and a compare circuit is programmed to activate an appropriate redundant select signal (RSEL.sub.0 to RSEL.sub.i). This programming is typically performed using fusible circuits. When an address of the defective column is provided on the address lines, the compare circuit responds by activating the redundant select signal to couple the redundant column to the appropriate data communication line.
Circuit designers continuously strive to achieve higher population capacities without a corresponding increase in physical size. Reducing the number of individual elements in integrated circuits is one way in which available die real estate is maximized. The above described redundancy scheme requires circuitry which adversely effects the available real estate and slows the operation of the memory.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a memory device which reduces the area and operating time penalty due to redundancy.